As a comment on the Motor Doctor feature on page 18 of the June 25 issue [“What You Need to Know About Motor Speed”], with 60 cycle-per-second voltage applied to a two-coil-only stator, half of the stator changes polarity from north to south to north 60 times per second. If the rotor consisting of short-circuited conductive bars is stationary, each half of the rotor is also subject to this change. But the strong magnetic field induced in the rotor directly opposes the stator field. So, despite heavy current draw, theoretically torque is zero.

If the rotor is spinning at 3,600 rpm, one half of the rotor faces from a north pole to another north pole in one half revolution to another north pole at one revolution, 60 times per second. The other rotor half faces only south poles.

Stator current is low. Rotor current is even lower, and again there is an absence of torque.

To overcome friction and windage, even an unloaded motor must have some loss of synchronous speed, or slip.

This results in stator or magnetic poles not aligned with rotor magnetic poles, which oppose the change, slowing, that produced them.

Stuart Welte

Welte Electric and Refrigeration

Sheldon, WI

Publication date: 07/23/2001